The present invention relates to a technology that is used for optically measuring a distance; and, more particularly, the invention relates to a distance measuring apparatus and an image input apparatus.
In order to obtain information concerning a three-dimensional shape and a position of an object in an image from the image taken by a camera, apparatuses for measuring the distance from the camera to each point in the image have been proposed and are commercially available.
As one of the methods of determining a distance using an image, various techniques have been proposed for measuring a distance using a relation between blur, caused by a positional relation between a lens and an imaging plane, and the distance to a camera target. As an example, some methods have been proposed which use a relation between various magnitudes of blur and distances, instead of directly obtaining an in-focus position of the lens. Among these methods, a method proposed by Professor Nayar of Columbia University uses a tele-centric optical system and a double focus camera. Further, in a technique disclosed in Japanese Patent No.2963990, it is proposed to use a structured coded opening (pupil) in order to make analysis of the blur easy.
As pointed out in Japanese Patent No.2963990, the method proposed by Nayar requires a modeling of the lens aberration, a precise positioning of the CCD (charge-coupled device) and removing of noise from an obtained image, because the method uses a simple circular opening lens. Further, it is difficult to measure an object located at a long distant position and to secure depth accuracy because the magnitude of the blur is limited to a comparatively small value.
On the other hand, in the method proposed by Japanese Patent No.2963990, a pupil-shaped light passing means for transmitting light is used so as to make an analysis of the magnitude of the blur easy. Thereby, it is possible to perform distance measurement without the occurrence of any problems in the above-mentioned method of Nayar.
In each of the above-mentioned methods, a distance measurement is performed by analyzing blurs in images taken at different focuses. As the means for taking the images at the different focuses, there are proposed a method in which a plurality of different focused images are taken several times by sequentially moving the lens to change the focus and a method in which a plurality of different focused images are taken at one time by dispersing the incident light in n directions using prisms.
In the method which calls for sequentially moving the lens, it is necessary to perform focus control with a high accuracy. On the other hand, in the method which calls for the use of prisms, the amount of light in the observed image at each focus becomes 1/n, because the incident light is divided into n parts. Accordingly, because of the reduced light level, the amount of charge obtained by the CCD is reduced. Therefore, this method is weak in avoiding electron noise, and so it is difficult to improve the measuring accuracy with this method.
As described above, in each of the conventional methods, there is a requirement to provide an operating accuracy of the focus control mechanism in the measurement apparatus that is extremely high. As a result, the satisfaction of the above requirement becomes an important problem for commercially using the measurement apparatus.